Chromatic dispersion limits the bandwidth of optical fibers by producing pulse spreading due to the various colors of light traveling in the fiber. Different wavelengths of light travel at different speeds; thus, since most optical sources emit light containing a range of wavelengths, each of these wavelengths arrive at a destination at different times, thereby causing the transmitted pulse to spread or "disperse" as it travels down the fiber.
Chromatic dispersion is the sum of material and waveguide dispersion of the fiber. Dispersion can be positive or negative because it measures the change in the refractive index with respect to the wavelength. Thus, the total chromatic dispersion can be zero (or approximately zero); the wavelength at which the chromatic dispersion is zero is known as the zero dispersion wavelength.
Accurate knowledge of dispersion is very important in high speed WDM optical links, because a pulse can travel essentially undistorted along the length of the fiber if the wavelength of the pulse can be matched to the zero-dispersion wavelength of the fiber. It is well known that the physical properties of fibers vary as the fiber is being drawn. This influences the optical properties, especially the dispersion. Several non-destructive techniques have been developed to measure the chromatic dispersion and the zero dispersion wavelength variations along the length of a fiber. One linear technique based on the use of an Optical Time Domain Reflectometry (OTDR) has been proposed for step-index fibers as described in an article in Electronics Letters 29, 426 (1993) by M. Ohashi and M. Tateda, incorporated by reference herein. Other reported techniques rely on the use of four-wave mixing (FWM) as a probe for the chromatic dispersion (D) or zero-dispersion wavelength (.lambda.0) fluctuation (see, for example, Y. Suetsugu, T. Kato, T. Okuno, and M. Nishimura, IEEE Phot. Lett. 7, 1459 (1995); S. Nishi and M. Saruwatari, Electron. Lett. 32, 579 (1996); and M. Eiselt, R. M. Jopson, and R. H. Stolen, J. Lightwave Technol. 15, 135 (1997), all of which are incorporated by reference herein). To date, the most effective technique uses the temporal oscillations in a backscattered FWM to measure the dispersion profile at a fixed wavelength, providing a spatial resolution of less than 500 m in the dispersion mapping. See. L. F. Mollenauer, P. V. Mamyshev, and M. J. Neubelt, Optics Letters 21, 1724 (1996), incorporated herein by reference.
The zero-dispersion wavelength spatial distribution can, in principle, be inferred from the profile measured at different wavelengths, and each of the prior art techniques described above is based on such imprecise inferential methods. There are many situations, however, which require exact, accurate information about the zero-dispersion wavelength distribution along the fiber length. For example, fiber devices which are based on the use of FWM usually require fiber having a very uniform zero-dispersion wavelength; thus, for such devices, accurate knowledge of the zero-dispersion wavelength along the length of the fiber is needed. None of the prior art non-destructive methods allow such a precise measurement of the zero dispersion wavelength.